1. Field of the Invention
This invention relates to reference generators, and more particularly to switchable current-reference voltage generators.
2. Description of Related Art
U.S. Pat. No. 4,814,688 discloses a reference generator which is either responsive to a constant current (current-controlled mode) or a reference voltage (voltage-controlled mode) in maintaining an energizing voltage at a substantially constant value. The system of the '688 patent isolates an operational amplifier 14 from the circuit during the current-controlled mode and prevents the reference voltage V.sub.ref provided by source 12 from affecting the operation of the reference generator 10.
A basic schematic of this type of reference generator is shown in FIG. 1. This reference generator includes an operational amplifier 2, switches 4 and 6, a resistor 8, a current source 10, a voltage source V.sub.CC, and a p-channel transistor 12. The voltage V.sub.BIAS is the reference output voltage which is used by the rest of the chip to generate current.
When switches 4A and 4B are at their "on" (i.e. closed) position and switches 6A and 6B are at their "off" (i.e. open) position, as shown, the circuit is in its "voltage-controlled mode". In this configuration, the output V.sub.BIAS of operational amplifier 2 is applied to the gate of transistor 12. Therefore, the gate to source voltage V.sub.GS of transistor 12 is represented by the equation EQU V.sub.GS =V.sub.G -V.sub.S ( 1)
where V.sub.G is the gate voltage and V.sub.S is the source voltage. It logically follows from the illustrated circuit that EQU V.sub.GS =V.sub.BIAS -V.sub.CC ( 2)
Transistor 12, a p-type MOSFET transistor, conducts when .vertline.V.sub.GS .vertline. exceeds the threshold voltage of transistor 12. Accordingly, current flows along a current path 14 that includes the voltage source V.sub.CC, the transistor 12, the resistor 8, and ground.
The voltage produced across the resistor 8 is introduced to the positive input of operational amplifier 2 which results in an output voltage appropriate to maintain substantially constant current through the path 14. Current through path 14 is calculated by the following equation: EQU Current=V.sub.BIAS /R.sub.8 ( 3)
in which R.sub.8 is the resistance of resistor 8.
When switches 4A and 4B are at their "off" position and switches 6A and 6B are at their "on" position, the circuit is in its "current-controlled mode". Since the output of operational amplifier 2 is disconnected from the gate of transistor 12, operational amplifier 2 has no effect on transistor 12. Instead, the gate of transistor 12 is connected by switch 6A to the drain of transistor 12, so that EQU V.sub.BIAS =V.sub.CC -V.sub.SD ( 4)
where V.sub.SD is the source-to-drain voltage. The current through path 14 is maintained by the constant current source 10.
In both cases, the voltage V.sub.BIAS is determined by the source-drain current of transistor 12.
One major drawback of switching the output of the operational amplifier 2 is that the operational amplifier 2 is "open loop" when switches 4A and 4B are "off". Because the operational amplifier is typically designed with a large voltage gain and bandwidth, this open loop condition causes adverse effects. For example, the open loop condition may cause the output to fluctuate in an exaggerated manner. This fluctuation draws current from the power line or the substrate. Drawing current in this manner causes noise which can propagate to other parts of the circuit, thereby reducing the circuit's performance predictability.